add pytest-mock to the hardcoded dependencies

this is workaround of python-poetry/poetry#3514 issue

.github/workflows/python_checker.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install flake8 flake8-docstrings pytest pytest-cov
+        pip install flake8 flake8-docstrings pytest pytest-cov pytest-mock
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
         if [ -f pyproject.toml ]; then pip install .; fi
     - name: Test with pytest